Coordinated Universal Time or UTC, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. Time zones around the world are expressed as positive and negative offsets from UTC. UTC is the successor of Greenwich Mean Time, abbreviated as GMT, and is still colloquially called GMT sometimes. The new name was coined to eliminate having the name of a specific location in an international standard. UTC bases time measurement on atomic standards rather than GMT's celestial ones.

Because the rotation of the Earth slows down, GMT lags behind atomic time, measured by atomic clocks. UTC is kept within 0.9 s of UT1; leap seconds are added (or, theoretically, subtracted) at the end of a month as necessary. To date all such adjustments have been positive, adding a leap second at 23:59:60. The issuing of leap seconds is determined by the International Earth Rotation Service, based on their measurements of the Earth's rotation.

International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the International Bureau of Weights and Measures (BIPM). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds.

UTC presents problems for computer systems such as Unix which store time as the number of seconds from a reference time. Because of leap seconds, it is impossible to determine the representation of a future date, because the number of leap seconds included in that date is unknown.

UTC is the time system used for many Internet and World Wide Web standards. In particular, the Network Time Protocol - NTP is designed as a way of dynamically distributing UTC time over the Internet.

There are some classes of software UTC clocks:

Relating to the calculation of the hour:

Drag when the clock shows the UTC hour calculating it from your local computer clock. You can see if a UTC clock is a drag one changing your local computer clock: if UTC hour varies, it is a drag UTC clock.

Autonomous, if it is not a drag clock. This is the best class of UTC clock.

Showing the hour:

Static: the time does not change from the latest reload.

Dynamic: the time changes from minute to minute

As indicated in the standards, it is convenient to include the UTC date too.

The UTC time zone is sometimes denoted by the letter 'Z' for military purposes. Since the NATO phonetic alphabet and radio-amateur word for 'Z' is "Zulu", UTC is sometimes known as Zulu time.

Wikipedia's own servers use Coordinated Universal Time, but logged-in users can set their time zone in the user preferences, see also Wikipedia's User Preferences Help: Textbox and time.